Power conversion system, photovoltaic characteristic curve scanning method and controller

Abstract

This application discloses a power conversion system, a method for scanning photovoltaic characteristic curves, and a controller. In off-grid operation, the power conversion system, including a first power conversion device and a second power conversion device, uses the second power conversion device to construct a grid voltage reference value to limit the initial MPPT voltage lower limit. Based on the initial MPPT voltage lower limit, the first power conversion device obtains a first segment of the photovoltaic characteristic curve of the first photovoltaic module. The second power conversion device then lowers the grid voltage reference value to a target grid voltage reference value. Based on the target MPPT voltage lower limit, the first power conversion device obtains a second segment of the photovoltaic characteristic curve. The first and second segments together form the photovoltaic characteristic curve of the first photovoltaic module. The scanned photovoltaic characteristic curve has a wider voltage range and is closer to the photovoltaic characteristic curve within the complete voltage range, improving the reliability and accuracy of fault detection for the photovoltaic module.

Description

Technical Field

[0001] This application relates to the field of power conversion technology, and more specifically, to a power conversion system, a method for scanning photovoltaic characteristic curves, and a controller. Background Technology

[0002] Against the backdrop of the rapid development of green energy, photovoltaic (PV) power generation has gradually become a crucial component. PV modules are the core component of power conversion systems, and their stability and reliability have a decisive impact on the overall system's operating efficiency. To ensure the efficient, stable, and reliable operation of PV modules, regular testing and evaluation of the modules are typically conducted based on the analysis of their photovoltaic characteristic curves.

[0003] In related technologies, the power conversion system typically needs to be in grid-connected operation, meaning it needs to be connected to the power grid to absorb electrical energy. This is achieved by using the Maximum Power Point Tracking (MPPT) function of the power conversion device within the system to obtain the photovoltaic (PV) characteristic curves of the PV modules (such as current-voltage (IV) or power-voltage (PV) characteristic curves). However, the PV characteristic curves obtained in this way are not complete enough, thus reducing the reliability and accuracy of PV module fault detection based on these curves. Summary of the Invention

[0004] This application provides a power conversion system, a method for scanning photovoltaic characteristic curves, and a controller. The various aspects involved in this application's embodiments are described below.

[0005] In a first aspect, a power conversion system is provided, including a first power conversion device and a second power conversion device. When the power conversion system is in an off-grid operation state, the power conversion system is configured to: construct a grid voltage reference value using the second power conversion device, the grid voltage reference value being used to define an initial MPPT voltage lower limit; perform a first MPPT scan using the first power conversion device based on the initial MPPT voltage lower limit to obtain a first photovoltaic characteristic curve of a first photovoltaic module; reduce the grid voltage reference value to a target grid voltage reference value using the second power conversion device, the target grid voltage reference value being used to adjust the initial MPPT voltage lower limit to the target MPPT voltage lower limit, the target MPPT voltage lower limit being less than the initial MPPT voltage lower limit; and perform a second MPPT scan using the first power conversion device based on the target MPPT voltage lower limit to obtain a second photovoltaic characteristic curve of the first photovoltaic module; wherein the first photovoltaic module is connected to the input terminal of the first power conversion device, and the first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module, the photovoltaic characteristic curve including an IV curve or a PV curve.

[0006] In some embodiments, the first MPPT scan corresponds to a first control of the operating voltage of the first power conversion device, the first control including controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to the initial MPPT voltage lower limit; the second MPPT scan corresponds to a second control of the operating voltage of the first power conversion device, the second control including controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to the target MPPT voltage lower limit.

[0007] In some embodiments, the number of second power conversion devices is multiple, and the power conversion system is further configured to: control the first power conversion device to replace at least one of the multiple second power conversion devices to jointly construct the grid voltage reference value with the remaining second power conversion devices, wherein the remaining second power conversion devices are the second power conversion devices other than the at least one second power conversion device among the multiple second power conversion devices; and obtain a first photovoltaic characteristic curve and a second photovoltaic characteristic curve of at least one second photovoltaic module based on the MPPT scan of the at least one second power conversion device, wherein the at least one second photovoltaic module is connected to the input terminal of the at least one second power conversion device respectively.

[0008] In some embodiments, the power conversion system further includes a grid-connected transformer and a grid-connected switch. A first end of the grid-connected transformer is connected to the output end of both the first power conversion device and the output end of the second power conversion device. A second end of the grid-connected transformer is used to connect to the public power grid through the grid-connected switch. The power conversion system is configured to: control the grid-connected switch to open and control the second power conversion device to run a virtual synchronous generator control strategy or a voltage-frequency control strategy so that the power conversion system has the grid voltage reference value when it is in an off-grid operation state.

[0009] In some embodiments, the grid voltage reference value is reduced by a first preset step size, and the operating voltage is reduced by a second preset step size.

[0010] In some embodiments, the first photovoltaic characteristic curve of the first photovoltaic module and the second photovoltaic characteristic curve of the first photovoltaic module are spliced ​​together based on the initial MPPT voltage lower limit value, and the current value of the first photovoltaic characteristic curve at the initial MPPT voltage lower limit value is equal to the current value of the second photovoltaic characteristic curve at the initial MPPT voltage lower limit value, so as to jointly form the photovoltaic characteristic curve of the first photovoltaic module.

[0011] In some embodiments, the first power conversion device and the second power conversion device each include: a DC-DC power conversion circuit and a DC-AC power conversion circuit, or a DC-DC power conversion circuit.

[0012] Secondly, a method for scanning a photovoltaic characteristic curve is provided. The method is applied to a power conversion system operating off-grid. The power conversion system includes a first power conversion device and a second power conversion device. The method includes: constructing a grid voltage reference value using the second power conversion device, the grid voltage reference value being used to initially define a lower limit of the MPPT voltage; performing a first segment MPPT scan using the first power conversion device based on the initial MPPT voltage lower limit to obtain a first segment photovoltaic characteristic curve of a first photovoltaic module; reducing the grid voltage reference value to a target grid voltage reference value using the second power conversion device, the target grid voltage reference value being used to adjust the initial MPPT voltage lower limit to the target MPPT voltage lower limit, the target MPPT voltage lower limit being less than the initial MPPT voltage lower limit; and performing a second segment MPPT scan using the first power conversion device based on the target MPPT voltage lower limit to obtain a second segment photovoltaic characteristic curve of the first photovoltaic module. The first photovoltaic module is connected to the input terminal of the first power conversion device, and the first segment photovoltaic characteristic curve and the second segment photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module, the photovoltaic characteristic curve including an IV curve or a PV curve.

[0013] In some embodiments, performing a first MPPT scan using the first power conversion device based on the initial MPPT voltage lower limit includes: performing a first control on the operating voltage of the first power conversion device, the first control including controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to the initial MPPT voltage lower limit; performing a second MPPT scan using the first power conversion device based on the target MPPT voltage lower limit includes: performing a second control on the operating voltage of the first power conversion device, the second control including controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to the target MPPT voltage lower limit.

[0014] In some embodiments, the number of the second power conversion devices is multiple, and the method further includes: controlling the first power conversion device to replace at least one of the multiple second power conversion devices to jointly construct the grid voltage reference value with the remaining second power conversion devices, wherein the remaining second power conversion devices are the second power conversion devices other than the at least one second power conversion device among the multiple second power conversion devices; and obtaining a first photovoltaic characteristic curve and a second photovoltaic characteristic curve of at least one second photovoltaic module based on the MPPT scan of the at least one second power conversion device, wherein the at least one second photovoltaic module is connected to the input terminal of the at least one second power conversion device respectively.

[0015] In some embodiments, the power conversion system further includes a grid-connected transformer and a grid-connected switch. A first end of the grid-connected transformer is connected to the output end of both the first power conversion device and the output end of the second power conversion device. A second end of the grid-connected transformer is used to connect to the public power grid through the grid-connected switch. The method further includes: controlling the grid-connected switch to open and controlling the second power conversion device to run a virtual synchronous generator control strategy or a voltage-frequency control strategy so that the power conversion system has the grid voltage reference value when it is in an off-grid operation state.

[0016] Thirdly, a controller is provided for performing the method as described in the second aspect.

[0017] Since the grid voltage reference value in this embodiment is adjustable, the lowest voltage covered by the photovoltaic characteristic curve of the first photovoltaic module finally scanned in this embodiment is the target MPPT voltage lower limit value, which is less than the initial MPPT voltage lower limit value. Therefore, compared with the related technology where the lowest voltage covered by the photovoltaic characteristic curve of the first photovoltaic module scanned is the initial MPPT voltage lower limit value, the voltage range of the photovoltaic characteristic curve of the first photovoltaic module scanned in this embodiment is larger and closer to the photovoltaic characteristic curve within the complete voltage range, thereby improving the reliability and accuracy of photovoltaic module fault detection. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the architecture of a vehicle remote control communication system in related technologies.

[0019] Figure 2 This is a schematic diagram of the architecture of a vehicle remote control communication system provided in one embodiment of this application.

[0020] Figure 3 This is a schematic flowchart of a vehicle remote control method provided in an embodiment of this application.

[0021] Figure 4 This is a flowchart illustrating a vehicle remote control method provided in another embodiment of this application.

[0022] Figure 5 This is a schematic diagram of the architecture of a vehicle remote control communication system provided in another embodiment of this application. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present application should fall within the scope of protection of the present application.

[0024] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0025] Against the backdrop of the rapid development of green energy, photovoltaic (PV) power generation has gradually become a crucial component. As a core component of the power conversion system, the stability and reliability of PV modules have a decisive impact on the overall system's operating efficiency. Because PV modules are exposed to the outdoor environment for extended periods, they are inevitably challenged by various external factors such as weather and environmental conditions. With prolonged use, the performance of PV strings may gradually degrade, such as reduced PV efficiency and increased internal resistance. Furthermore, potential faults within the PV modules, such as cell breakage, damaged wiring, and contamination, will ultimately affect their stability and reliability. Therefore, regular testing and evaluation of PV modules are essential to ensure their efficient, stable, and reliable operation. This process not only helps us identify potential problems in the modules in a timely manner but also ensures that the PV modules can maintain a high power generation rate, thereby ensuring the stable operation of the entire power conversion system.

[0026] In current testing and evaluation technologies, analyzing the photovoltaic (PV) characteristic curves of PV modules is a common method for regular testing and evaluation. Specifically, PV module failures directly affect the shape and position of the PV characteristic curve, leading to anomalies. Therefore, by periodically scanning the PV characteristic curve, the performance degradation of the PV module can be monitored. By comparing the actual measured PV characteristic curve with the theoretical PV characteristic curve or historical data, problems can be quickly identified and located, allowing for timely maintenance or replacement.

[0027] In related technologies, the power conversion system typically needs to be in grid-connected operation, meaning it must be connected to the power grid to absorb electrical energy and obtain the photovoltaic characteristic curve of the photovoltaic module based on the MPPT scanning function of the power conversion device within the system. However, due to the influence of AC side voltage level and multi-level modulation during grid-connected operation, the DC side voltage of the power conversion device within the power conversion system needs to be limited, preventing the DC side voltage from reaching a low level. Therefore, the initial MPPT voltage lower limit cannot reach a low level. Consequently, the photovoltaic characteristic curve scanned in grid-connected operation in related technologies is incomplete due to the lack of a low-voltage segment, thus reducing the reliability and accuracy of photovoltaic module fault detection based on this photovoltaic characteristic curve.

[0028] In view of this, embodiments of this application provide a power conversion system, which includes a first power conversion device and a second power conversion device. When the power conversion system is in an off-grid operation state, the second power conversion device can be used to construct a grid voltage reference value, which is used to limit an initial MPPT voltage lower limit. The first power conversion device performs a first MPPT scan based on the initial MPPT voltage lower limit to obtain a first photovoltaic characteristic curve of the first photovoltaic module. Then, the second power conversion device reduces the grid voltage reference value to a target grid voltage reference value, and the first power conversion device performs a second MPPT scan based on the target MPPT voltage lower limit to obtain a second photovoltaic characteristic curve of the first photovoltaic module. The target grid voltage reference value is used to adjust the initial MPPT voltage lower limit to the target MPPT voltage lower limit, and the target MPPT voltage lower limit is less than the initial MPPT voltage lower limit. The first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module. Since the grid voltage reference value in this embodiment is adjustable, the lowest voltage covered by the photovoltaic characteristic curve of the first photovoltaic module finally scanned in this embodiment is the target MPPT voltage lower limit value, which is less than the initial MPPT voltage lower limit value. Therefore, compared with the related technology where the lowest voltage covered by the photovoltaic characteristic curve of the first photovoltaic module scanned is the initial MPPT voltage lower limit value, the voltage range of the photovoltaic characteristic curve of the first photovoltaic module scanned in this embodiment is larger and closer to the photovoltaic characteristic curve within the complete voltage range, thereby improving the reliability and accuracy of photovoltaic module fault detection.

[0029] The following is combined Figure 1 and Figure 2 The power conversion system provided in this application embodiment will be described in more detail. It should be noted that the power conversion system provided in this application embodiment can be a power supply system capable of converting direct current (DC) generated by photovoltaic modules into alternating current (AC). The power conversion system can be in grid-connected operation or off-grid operation. The photovoltaic characteristic curve scanning scheme involved in this application embodiment is an online photovoltaic characteristic curve scanning scheme applied to a power conversion system in off-grid operation.

[0030] Grid-connected operation means that the power conversion system can be connected to the public power grid (or mains grid) to supply power to the load together with the public grid. In this case, the voltage and frequency output by the power conversion system need to follow the public grid. Off-grid operation means that the power conversion system can be disconnected from the public grid and supply power to the load independently by relying on the power conversion devices within the power conversion system. In this case, the power conversion system establishes and maintains its own voltage and frequency. In some embodiments, when the power conversion system is operating off-grid, the grid voltage reference value constructed by the power conversion devices within the power conversion system can be called the microgrid voltage reference value.

[0031] like Figure 1 As shown, the power conversion system may include a first power conversion device and a second power conversion device. The input terminal of the first power conversion device is connected to a first photovoltaic module, and the input terminal of the second power conversion device is connected to a second photovoltaic module. Both the first and second power conversion devices have MPPT (Multi-Level Photovoltaic Test) functionality, MPPT scanning functionality, grid construction functionality, and the ability to convert the direct current generated by the corresponding photovoltaic module into electrical current.

[0032] MPPT (Maximum Power Point Test) can be understood as a control algorithm and hardware coordination mechanism used to adjust the operating point of multiple connected photovoltaic modules in the power converter in real time, ensuring that they always operate at the maximum power output state under the current light / temperature conditions. MPPT corresponds to the MPPT voltage lower limit, which is the minimum DC input voltage required for the power converter to activate and maintain the MPPT function. If the operating voltage of the power converter is lower than the MPPT voltage lower limit, the MPPT function will not work, meaning the power converter cannot operate normally, and the corresponding power conversion system may shut down or enter standby mode.

[0033] The MPPT scanning function can be understood as a function that, when the regular MPPT function is paused, systematically adjusts the operating voltage of the power conversion device to synchronously acquire IV or PV data and generate photovoltaic characteristic curves. The photovoltaic characteristic curves include either the IV curve or the PV curve. Since the MPPT voltage lower limit is the minimum operating voltage of the power conversion device during normal operation, the minimum scanning voltage during MPPT scanning is also the MPPT voltage lower limit. The grid voltage reference value is used to limit the MPPT voltage lower limit. Specifically, the MPPT voltage lower limit is positively correlated with the grid voltage reference value; for example, the larger the grid voltage reference value, the larger the MPPT voltage lower limit, and vice versa.

[0034] Therefore, if the power conversion system is operating in grid-connected mode, the grid voltage reference value that limits the MPPT lower voltage limit is the voltage reference value of the public grid. Since the voltage reference value of the public grid is constant, the MPPT lower voltage limit cannot be changed. It should be noted that the MPPT lower voltage limit defined by the voltage reference value of the public grid can be understood as the initial MPPT lower voltage limit mentioned below. Therefore, when the power conversion system is operating in grid-connected mode, the initial MPPT lower voltage limit cannot be changed. However, if the power conversion system is operating off-grid, since the power conversion system can build its own power supply network similar to a grid, and the grid voltage reference value provided by this network is variable, the initial MPPT lower voltage limit can be varied under this premise.

[0035] for Figure 1 The power conversion system shown is configured to perform the following when it is in off-grid operation: Figure 2 The method shown. See details. Figure 2 The method performed by a photovoltaic power conversion system in off-grid operation may include steps S210-S240.

[0036] In step S210, a grid voltage reference value is constructed using the second power conversion device.

[0037] The grid voltage reference value is an AC supply voltage similar to the voltage reference value of the public power grid, used to complete the energy absorption during the first MPPT scan (or initial MPPT scan) performed by the first power conversion device. Based on this, the grid voltage reference value is used to define the initial MPPT voltage lower limit. Constructing a grid voltage reference value using a second power conversion device can be understood as establishing a grid voltage reference value using the second power conversion device. The number of second power conversion devices can be one or more. It should be noted that when there are multiple second power conversion devices, the established grid voltage reference value can better meet the practical engineering requirements of high power, high reliability, and scalability.

[0038] This application does not specifically limit the method by which the second power conversion device constructs the grid voltage reference value. For example, the second power conversion device can operate a virtual synchronous generator (VSG) to construct the grid voltage reference value from the master. Alternatively, the second power conversion device can operate a voltage and frequency (VF) control strategy to construct the grid voltage reference value from the master.

[0039] In step S220, the first power conversion device performs a first MPPT scan based on the initial MPPT voltage lower limit to obtain the first photovoltaic characteristic curve of the first photovoltaic module.

[0040] The first photovoltaic characteristic curve of the first photovoltaic module is the photovoltaic characteristic curve of the first photovoltaic module obtained after the first power conversion device performs the first MPPT scan. The principle of the first MPPT scan can be found in the MPPT scan section above, except that for the first MPPT scan, the lowest scan voltage during the first MPPT scan is the initial MPPT voltage lower limit. Therefore, the lowest voltage corresponding to the first photovoltaic characteristic curve of the first photovoltaic module is the initial MPPT voltage lower limit.

[0041] In step S230, the second power conversion device is used to reduce the grid voltage reference value to the target grid voltage reference value.

[0042] Step S230 can be understood as follows: Based on the control algorithm within the second power conversion device, the grid voltage reference value constructed in step S210 is reduced, so that the reduced grid voltage reference value is the target grid voltage reference value, and the target grid voltage reference value is less than the grid voltage reference value constructed in step S210. The target grid voltage reference value is used to adjust the initial MPPT voltage lower limit to the target MPPT voltage lower limit; in other words, the MPPT voltage lower limit defined by the target grid voltage reference value can be called the target MPPT voltage lower limit. Given that the grid voltage reference value and the MPPT voltage lower limit are positively correlated, when the grid voltage is reduced to the target grid voltage reference value, the initial MPPT voltage lower limit can be dynamically adjusted to the target MPPT voltage lower limit, and the target MPPT voltage lower limit is less than the initial MPPT voltage lower limit.

[0043] In step S240, the first power conversion device performs a second MPPT scan based on the target MPPT voltage lower limit to obtain the second photovoltaic characteristic curve of the first photovoltaic module.

[0044] The second photovoltaic characteristic curve of the first photovoltaic module is the other photovoltaic characteristic curve of the first photovoltaic module obtained after the first power conversion device performs the second MPPT scan. The principle of the second MPPT scan can be found in the MPPT scan section above, except that for the second MPPT scan, the lowest scan voltage during the second MPPT scan is the target MPPT voltage lower limit. Therefore, the lowest voltage corresponding to the second photovoltaic characteristic curve of the first photovoltaic module is the target MPPT voltage lower limit.

[0045] In this design, the first segment of the photovoltaic characteristic curve and the second segment of the photovoltaic characteristic curve of the first photovoltaic module are two different photovoltaic characteristic curves, and these two photovoltaic characteristic curves together form the photovoltaic characteristic curve of the first photovoltaic module. As an example, taking the photovoltaic characteristic curve as the PV curve, the first segment of the photovoltaic characteristic curve of the first photovoltaic module is... Figure 3 In the PV curve of segment a, the second segment of the photovoltaic characteristic curve of the first photovoltaic module is... Figure 3 In the b-segment PV curve, the photovoltaic characteristic curve of the first photovoltaic module is the a-segment PV curve + the b-segment PV curve. In other words, the PV curve of the first photovoltaic module finally scanned in this application embodiment is the a-segment PV curve + the b-segment PV curve.

[0046] It should be understood that Figure 3 The PV curve in segment a is the PV curve of the first photovoltaic module obtained by scanning during grid-connected operation using relevant technologies. Figure 3 A comparison of the PV curve segment a and the PV curve segment a + segment b shows that the voltage range of the PV curve segment a is relatively small, and it cannot scan the voltage range below the initial MPPT voltage lower limit. However, the PV curve of the first photovoltaic module scanned in this embodiment can scan the voltage range below the initial MPPT voltage lower limit. Therefore, the scanned voltage range is larger and closer to scanning the photovoltaic characteristic curve within the complete voltage range. Given the possibility of multiple peaks in the photovoltaic characteristic curve, a larger scanned voltage range is more conducive to improving the reliability and accuracy of fault detection of photovoltaic modules.

[0047] Furthermore, the embodiments of this application only require the power conversion system to perform corresponding control on the first and second power conversion devices when it is in off-grid operation, which can achieve the scanning of photovoltaic characteristic curves over a wider voltage range. No additional hardware or modification of the power conversion devices is required. Therefore, the implementation method is simple and the cost is low.

[0048] This application does not specifically limit the target MPPT voltage lower limit value, as long as the target MPPT voltage lower limit value is less than the initial MPPT voltage lower limit value. Optionally, the target MPPT voltage lower limit value is less than the initial MPPT voltage lower limit value and there is no peak value on the photovoltaic characteristic curve between 0 and the target MPPT voltage lower limit value. In some embodiments, such as Figure 4 As shown, in order to scan the complete photovoltaic characteristic curve, the target MPPT voltage lower limit can be close to 0.

[0049] It should be noted that, for ease of illustration, in Figures 3-4 In the diagram, the open-circuit voltage is labeled V1, the initial MPPT voltage lower limit is labeled V2, and the target MPPT voltage lower limit is labeled V3.

[0050] In some embodiments, the first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module. This can be achieved by splicing the first and second photovoltaic characteristic curves of the first photovoltaic module. The splicing can be seamless, or in other words, splicing based on the principle of voltage-current continuity. Specifically, it can be as follows: Figures 3-4 As shown, the first and second photovoltaic characteristic curves of the first photovoltaic module are spliced ​​together based on the initial MPPT voltage lower limit. Furthermore, the current value of the first photovoltaic characteristic curve at the initial MPPT voltage lower limit is equal to the current value of the second photovoltaic characteristic curve at the initial MPPT voltage lower limit, thus forming the photovoltaic characteristic curve of the first photovoltaic module. By splicing based on a dual matching of "voltage and current" at the initial MPPT voltage lower limit, a seamless splicing of the first and second photovoltaic characteristic curves of the first photovoltaic module can be achieved, resulting in a complete and redundant photovoltaic characteristic curve for the first photovoltaic module.

[0051] This application does not specifically limit the structure of the first power conversion device and / or the second power conversion device, as long as they can have the above-mentioned functions. For example, both the first and second power conversion devices include a direct current to alternating current (DC-AC) power conversion circuit. As a specific example, both the first and second power conversion devices specifically include a DC-AC converter, and this DC-AC converter can be connected to an MPPT boost combiner box. The MPPT boost combiner box, as a direct current to direct current (DC-DC) power conversion circuit boost converter, can include at least two DC-DC circuits, and the input terminal of each DC-DC circuit is connected to a photovoltaic module. The MPPT scanning technology applied to this power conversion system can use the DC-DC circuit in the MPPT boost combiner box to control the output voltage of the photovoltaic module, thereby realizing the scanning of the photovoltaic characteristic curve of the corresponding photovoltaic module.

[0052] For example, both the first and second power conversion devices include DC-AC and DC-DC converters. As a specific example, both the first and second power conversion devices specifically include DC-DC + DC-AC, where there can be multiple DC-DC converters. The input terminals of these multiple DC-DC circuits are connected to multiple photovoltaic modules, and the output terminals of these multiple DC-DC circuits are connected in parallel to the DC-AC circuit. The MPPT scanning technology applied to this power conversion system, by controlling the input voltage of the DC-DC circuits, effectively scans the output voltage of the connected photovoltaic modules, thus achieving the scanning of the photovoltaic characteristic curves of the corresponding photovoltaic modules.

[0053] As mentioned earlier, MPPT scanning of a power conversion device requires systematic adjustment of its operating voltage. Therefore, in some embodiments, the first MPPT scan corresponds to a first control of the operating voltage of the first power conversion device. This first control includes controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to an initial MPPT voltage lower limit. The first MPPT scan, during the first control process, simultaneously collects IV or PV data to generate a first segment of the photovoltaic characteristic curve of the first photovoltaic module. The second MPPT scan corresponds to a second control of the operating voltage of the first power conversion device. This second control includes controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to a target MPPT voltage lower limit. The second MPPT scan, during the second control process, simultaneously collects IV or PV data to generate a second segment of the photovoltaic characteristic curve of the first photovoltaic module. Performing the first and second MPPT scans based on the first and second controls respectively can avoid repeated scanning while ensuring a relatively complete photovoltaic characteristic curve of the first photovoltaic module is obtained, thereby improving the efficiency of obtaining a relatively complete photovoltaic characteristic curve of the first photovoltaic module.

[0054] As mentioned above, both the first power conversion device and the second power conversion device are power conversion devices in a power conversion system. In order to perform MPPT scanning on each power conversion device in the power conversion system so that fault detection can be performed on the photovoltaic modules connected to each power conversion device, in some embodiments, after completing the MPPT scan on the first power conversion device, the roles of the first power conversion device and the second power conversion device can be switched so that the first power conversion device participates in the construction or control of the grid voltage reference value, and the second power conversion device performs the MPPT scan.

[0055] Specifically, the number of second power conversion devices can be multiple, and the power conversion system is further configured to: control the first power conversion device to replace at least one of the multiple second power conversion devices to jointly construct the grid voltage reference value with the remaining second power conversion devices; and obtain the first segment of photovoltaic characteristic curve and the second segment of photovoltaic characteristic curve of at least one second photovoltaic module based on the MPPT scan of at least one second power conversion device.

[0056] The remaining second power conversion device is a second power conversion device other than at least one second power conversion device among a plurality of second power conversion devices. That is, at least one second power conversion device and the remaining second power conversion devices together form a plurality of second power conversion devices. The at least one second photovoltaic module is a photovoltaic module connected to at least one second power conversion device. That is, at least one second photovoltaic module is connected to the input terminal of at least one second power conversion device.

[0057] In some embodiments, the number of first power conversion devices may be the same as the number of at least one second power conversion device to ensure that the role switching of the first power conversion device and the role switching of at least one second power conversion device are smooth, thereby improving the stability of the grid voltage reference value and the stability of the photovoltaic characteristic curve scanning.

[0058] In some embodiments, controlling a first power conversion device to replace at least one of a plurality of second power conversion devices to jointly construct a grid voltage reference value with the remaining second power conversion devices; obtaining a first segment photovoltaic characteristic curve and a second segment photovoltaic characteristic curve of at least one second photovoltaic module based on the MPPT scan of at least one second power conversion device, may specifically include: constructing a grid voltage reference value using the first power conversion device and the remaining second power conversion devices; performing a first segment MPPT scan using at least one second power conversion device based on an initial MPPT voltage lower limit value to obtain the first segment photovoltaic characteristic curve of at least one second photovoltaic module; reducing the grid voltage reference value to a target grid voltage reference value using the first power conversion device and the remaining second power conversion devices; and performing a second segment MPPT scan using at least one second power conversion device based on the target MPPT voltage lower limit value to obtain the second segment photovoltaic characteristic curve of at least one second photovoltaic module. The target grid voltage value is consistent with the above description and will not be repeated here.

[0059] The first and second MPPT scans here are consistent with the first and second MPPT scans corresponding to the first power conversion device mentioned earlier, except that the control of the operating voltage corresponding to the first and second MPPT scans here is the control of the operating voltage of at least one second power conversion device. It should be noted that the operating voltage of the power conversion device in this embodiment can be considered as the output voltage of the corresponding photovoltaic module. Therefore, the operating voltage of the first power conversion device can be understood as the output voltage of the first photovoltaic module, and the operating voltage of the second power conversion device can be understood as the output voltage of the second photovoltaic module.

[0060] To better understand the above-mentioned role-switching scheme, the following will combine... Figure 5 This will be illustrated with examples. See details. Figure 5 The power conversion system comprises four power conversion devices: power conversion device 1, power conversion device 2, power conversion device 3, and power conversion device 4. The input terminals of power conversion devices 1, 2, 3, and 4 are respectively connected to photovoltaic modules 1, 2, 3, and 4. Power conversion devices 1, 2, and 3 are second power conversion devices, photovoltaic modules 1, 2, and 3 are second photovoltaic modules, and power conversion device 4 is the first power conversion device, with photovoltaic module 4 being the first photovoltaic module.

[0061] Optionally, such as Figure 5 As shown, the power conversion system may further include a grid-connected transformer and a grid-connected switch. The first terminal of the grid-connected transformer is connected to the output terminals of both the first and second power conversion devices, while the second terminal is used to connect to the public power grid via the grid-connected switch. The grid-connected transformer can also be referred to as a medium-voltage switchgear, used to convert the output AC power from the first and / or second power conversion devices to AC power of a higher voltage level and to achieve electrical isolation. The grid-connected switch controls whether the power conversion system is connected to the public power grid. Specifically, when the grid-connected switch is closed, the power conversion system is connected to the public power grid, thus operating in grid-connected mode; when the grid-connected switch is open, the power conversion system is disconnected from the public power grid, thus operating in off-grid mode.

[0062] For a power conversion system that includes a grid-connected transformer and a grid-connected switch, the power conversion system can be configured to: control the grid-connected switch to open and control the second power conversion device to run a virtual synchronous generator control strategy or a voltage and frequency control strategy so that a grid voltage reference value exists when the power conversion system is in an off-grid operation state.

[0063] for Figure 5The power conversion system shown herein utilizes the photovoltaic characteristic curve scanning technology described in this application. The specific process is as follows: when the entire power conversion system is under maintenance, the grid-connected switch is disconnected; a microgrid is constructed by operating the second power conversion device (i.e., power conversion device 1, power conversion device 2, and power conversion device 3) in VSG or VF mode, which has a grid voltage reference value; a global MPPT scan is performed using the first power conversion device (i.e., power conversion device 4) to complete the full photovoltaic characteristic scan of the first photovoltaic module 4; after completing the global MPPT scan of the first power conversion device, the microgrid is further constructed using the VSG or VF modes of power conversion devices 1, 2, and 4; a global MPPT scan is performed using power conversion device 3 to complete the full photovoltaic characteristic scan of the second photovoltaic module 3 connected to power conversion device 3; and so on, to complete the off-grid photovoltaic characteristic curve scan of photovoltaic modules 1 and 2 connected to power conversion devices 1 and 2.

[0064] The global MPPT scan includes the first and second MPPT scans described above. Specifically, the global MPPT scan using the first power conversion device (i.e., power conversion device 4) involves: firstly, controlling the operating voltage of power conversion device 4 to gradually change from the open-circuit voltage to the initial MPPT voltage lower limit, completing this segment of the photovoltaic characteristic curve scan, i.e., the first MPPT scan; after completing the first MPPT scan, the grid voltage reference value is gradually lowered to the target grid voltage using the second power conversion devices (i.e., power conversion device 1, power conversion device 2, and power conversion device 3) connected to the power grid, while simultaneously reducing the operating voltage of power conversion device 4 to gradually change to the target MPPT voltage lower limit, completing this segment of the photovoltaic characteristic curve scan, i.e., the second MPPT scan. This achieves a complete scan of the photovoltaic characteristic curve of the photovoltaic modules connected to power conversion device 4, enabling fault diagnosis of the photovoltaic modules. The principle for global MPPT scanning of each second power conversion device is similar and will not be elaborated here.

[0065] According to existing photovoltaic characteristic curve scanning methods, the power conversion system must be connected to the public grid when the grid-connected switch is closed to perform photovoltaic characteristic curve scanning. By implementing the embodiments of this application, when the grid-connected switch is open, a grid voltage reference value can be constructed by operating multiple power converters in VSG / VF mode. This allows for photovoltaic characteristic curve scanning of other power converters even when disconnected from the actual public grid. Furthermore, since this grid voltage reference value is adjustable, the AC side voltage can be reduced, allowing the photovoltaic characteristic curve to be scanned at a lower voltage. This results in a more complete photovoltaic characteristic curve, solving the problem of inaccurate diagnosis of multi-peak photovoltaic characteristic curves and effectively meeting the needs of actual engineering operations. Additionally, since the grid-connected switch is open, this application avoids power fluctuations in the power generation system caused by the photovoltaic characteristic curve scanning process.

[0066] It should be noted that, Figure 5 The power conversion system described herein is an exemplary structure, provided only to facilitate understanding of the solution presented in this application, and should not be construed as limiting the scope of this application. This application can also reasonably recombine the solutions described above, and such recombination should also be included within the protection scope of this application.

[0067] As mentioned earlier, the second MPPT scan requires lowering the grid voltage reference value, and the MPPT scan process also requires lowering the operating voltage. This application does not specify the method for lowering the grid voltage reference value or the operating voltage, as long as the reduction in both the grid voltage reference value and the operating voltage is relatively smooth to ensure system stability.

[0068] As an example, the grid voltage reference value is reduced at a first preset slope, and the operating voltage is reduced at a second preset slope. In this way, the grid voltage reference value and the operating voltage can be reduced relatively quickly, thereby improving the system's response speed.

[0069] As another example, the grid voltage reference value is decreased by a first preset step size, and the operating voltage is decreased by a second preset step size. The first or second preset step size can be any voltage value, specifically related to the accuracy of the photovoltaic characteristic curve and / or the stability characteristics of the power conversion system. For example, the first preset step size could be 5V, and the second preset step size could be 1V.

[0070] The foregoing description, in conjunction with a detailed description of the apparatus embodiments of this application, also provides a method for scanning photovoltaic characteristic curves. It should be understood that the descriptions of the method embodiments correspond to the descriptions of the apparatus embodiments; therefore, any parts not described in detail can be referred to the foregoing apparatus embodiments.

[0071] like Figure 2As shown, the photovoltaic characteristic curve scanning method S200 provided in this application embodiment includes steps S210-S240. See the preceding text for details. It should be noted that the photovoltaic characteristic curve scanning method is applied to a power conversion system in off-grid operation, and the power conversion system includes a first power conversion device and a second power conversion device.

[0072] See details Figure 2 In step S210, a grid voltage reference value is constructed using the second power conversion device. The grid voltage reference value is used to define the initial MPPT voltage lower limit.

[0073] In step S220, the first power conversion device performs a first MPPT scan based on the initial MPPT voltage lower limit to obtain the first photovoltaic characteristic curve of the first photovoltaic module.

[0074] In step S230, the second power conversion device is used to reduce the grid voltage reference value to the target grid voltage. The target grid voltage reference value is used to adjust the initial MPPT voltage lower limit to the target MPPT voltage lower limit, where the target MPPT voltage lower limit is lower than the initial MPPT voltage lower limit.

[0075] In step S240, the first power conversion device performs a second MPPT scan based on the target MPPT voltage lower limit to obtain the second photovoltaic characteristic curve of the first photovoltaic module.

[0076] The first photovoltaic module is connected to the input terminal of the first power conversion device. The first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module. The photovoltaic characteristic curve includes the IV curve or the PV curve.

[0077] In some embodiments, performing a first MPPT scan using the first power conversion device based on an initial MPPT voltage lower limit includes: performing a first control on the operating voltage of the first power conversion device, the first control including controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to the initial MPPT voltage lower limit; performing a second MPPT scan using the first power conversion device based on a target MPPT voltage lower limit includes: performing a second control on the operating voltage of the first power conversion device, the second control including controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to the target MPPT voltage lower limit.

[0078] In some embodiments, the number of second power conversion devices is multiple, and the method further includes: controlling a first power conversion device to replace at least one of the multiple second power conversion devices to jointly construct a grid voltage reference value with the remaining second power conversion devices, wherein the remaining second power conversion devices are the second power conversion devices other than at least one of the multiple second power conversion devices; and obtaining a first photovoltaic characteristic curve and a second photovoltaic characteristic curve of at least one second photovoltaic module based on the MPPT scan of at least one second power conversion device, wherein at least one second photovoltaic module is connected to the input terminal of at least one second power conversion device respectively.

[0079] In some embodiments, the power conversion system further includes a grid-connected transformer and a grid-connected switch. The first end of the grid-connected transformer is connected to the output end of both the first power conversion device and the output end of the second power conversion device. The second end of the grid-connected transformer is used to connect to the public power grid through the grid-connected switch. The method further includes: controlling the grid-connected switch to open and controlling the second power conversion device to run a virtual synchronous generator control strategy or a voltage-frequency control strategy so that a grid voltage reference value exists when the power conversion system is in an off-grid operation state.

[0080] In some embodiments, the grid voltage reference value is reduced by a first preset step size, and the operating voltage is reduced by a second preset step size.

[0081] In some embodiments, the first photovoltaic characteristic curve of the first photovoltaic module and the second photovoltaic characteristic curve of the first photovoltaic module are spliced ​​together based on the initial MPPT voltage lower limit value, and the current value of the first photovoltaic characteristic curve at the initial MPPT voltage lower limit value is equal to the current value of the second photovoltaic characteristic curve at the initial MPPT voltage lower limit value, so as to jointly form the photovoltaic characteristic curve of the first photovoltaic module.

[0082] In some embodiments, the first power conversion device and the second power conversion device each include: a DC-DC power conversion circuit and a DC-AC power conversion circuit, or a DC-DC power conversion circuit.

[0083] This application also provides a controller that can be used to execute the methods described in the above method embodiments. This application does not limit the specific implementation of the controller. For example, the controller can be implemented in hardware when executing the above methods. Alternatively, the controller can be implemented using a combination of software and hardware. Yet another example is that the controller can be implemented in software, such as by an MCU running a computer program to execute the above methods.

[0084] This application also provides a chip, including a processor, which can be used to call and run a computer program from memory, causing a power converter or power system on which the chip is installed to perform the methods described in the above method embodiments. It is understood that the processor can be any type of processor mentioned above. It is also understood that the memory can be independent of the chip or integrated into the chip.

[0085] This application also provides a machine-readable storage medium for storing a program. This program causes a computer to execute the methods described in the various embodiments of this application.

[0086] This application also provides a computer program product. The computer program product includes a program. The program causes a computer to perform the methods described in various embodiments of this application.

[0087] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any other combination. When implemented in software, it can be implemented, in whole or in part, as a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this disclosure are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a machine-readable storage medium or transmitted from one machine-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another via wired (e.g., coaxial cable, fiber optic, Digital Subscriber Line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The machine-readable storage medium can be any available medium accessible to a computer or a data storage device such as a server or data center that integrates one or more available media. The available media can be magnetic media (e.g., floppy disks, hard disks, magnetic tapes), optical media (e.g., digital video discs (DVDs)), or semiconductor media (e.g., solid-state drives (SSDs)).

[0088] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments of this disclosure can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this disclosure.

[0089] In the several embodiments provided in this disclosure, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0090] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0091] In addition, the functional units in the various embodiments of this disclosure can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0092] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A power conversion system, characterized in that, The system includes a first power conversion device and a second power conversion device. When the power conversion system is in off-grid operation, the power conversion system is configured as follows: The second power conversion device is used to construct a grid voltage reference value, which is used to define the initial MPPT voltage lower limit value. The first power conversion device performs a first MPPT scan based on the initial MPPT voltage lower limit to obtain the first photovoltaic characteristic curve of the first photovoltaic module. The second power conversion device is used to reduce the grid voltage reference value to a target grid voltage reference value. The target grid voltage reference value is used to adjust the initial MPPT voltage lower limit value to the target MPPT voltage lower limit value, and the target MPPT voltage lower limit value is less than the initial MPPT voltage lower limit value. The first power conversion device performs a second MPPT scan based on the target MPPT voltage lower limit to obtain the second photovoltaic characteristic curve of the first photovoltaic module; The first photovoltaic module is connected to the input terminal of the first power conversion device. The first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module. The photovoltaic characteristic curve includes an IV curve or a PV curve.

2. The power conversion system according to claim 1, characterized in that, The first MPPT scan corresponds to a first control on the operating voltage of the first power conversion device. The first control includes controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to the initial MPPT voltage lower limit. The second MPPT scan corresponds to a second control on the operating voltage of the first power conversion device. The second control includes controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to the target MPPT voltage lower limit.

3. The power conversion system according to claim 1, characterized in that, The number of the second power conversion devices is multiple, and the power conversion system is further configured as follows: The first power conversion device is controlled to replace at least one of the plurality of second power conversion devices to jointly construct the grid voltage reference value with the remaining second power conversion devices, wherein the remaining second power conversion devices are the second power conversion devices other than the at least one second power conversion device among the plurality of second power conversion devices; Based on the MPPT scan of the at least one second power conversion device, a first photovoltaic characteristic curve and a second photovoltaic characteristic curve of the at least one second photovoltaic module are obtained, wherein the at least one second photovoltaic module is connected to the input terminal of the at least one second power conversion device.

4. The power conversion system according to claim 1, characterized in that, The power conversion system further includes a grid-connected transformer and a grid-connected switch. A first terminal of the grid-connected transformer is connected to the output terminals of both the first and second power conversion devices. A second terminal of the grid-connected transformer is used to connect to the public power grid via the grid-connected switch. The power conversion system is configured as follows: The grid-connected switch is disconnected, and the second power conversion device is controlled to run a virtual synchronous generator control strategy or a voltage and frequency control strategy so that the grid voltage reference value exists when the power conversion system is in off-grid operation.

5. The power conversion system according to claim 2, characterized in that, The grid voltage reference value is reduced by a first preset step size, and the operating voltage is reduced by a second preset step size.

6. The power conversion system according to any one of claims 1-5, characterized in that, The first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module are spliced ​​together based on the initial MPPT voltage lower limit value. The current value of the first photovoltaic characteristic curve at the initial MPPT voltage lower limit value is equal to the current value of the second photovoltaic characteristic curve at the initial MPPT voltage lower limit value, so as to jointly form the photovoltaic characteristic curve of the first photovoltaic module.

7. The power conversion system according to any one of claims 1-5, characterized in that, Both the first power conversion device and the second power conversion device include: a DC-DC power conversion circuit and a DC-AC power conversion circuit, or a DC-DC power conversion circuit.

8. A method for scanning photovoltaic characteristic curves, characterized in that, The method is applied to a power conversion system operating off-grid, the power conversion system including a first power conversion device and a second power conversion device, and the method includes: The second power conversion device is used to construct a grid voltage reference value, which is used to initially define the lower limit of the MPPT voltage. The first power conversion device performs a first MPPT scan based on the initial MPPT voltage lower limit to obtain the first photovoltaic characteristic curve of the first photovoltaic module. The second power conversion device is used to reduce the grid voltage reference value to a target grid voltage reference value. The target grid voltage reference value is used to adjust the initial MPPT voltage lower limit value to the target MPPT voltage lower limit value, and the target MPPT voltage lower limit value is less than the initial MPPT voltage lower limit value. The first power conversion device performs a second MPPT scan based on the target MPPT voltage lower limit to obtain the second photovoltaic characteristic curve of the first photovoltaic module; The first photovoltaic module is connected to the input terminal of the first power conversion device. The first photovoltaic characteristic curve and the second photovoltaic characteristic curve of the first photovoltaic module together form the photovoltaic characteristic curve of the first photovoltaic module. The photovoltaic characteristic curve includes an IV curve or a PV curve.

9. The method according to claim 8, characterized in that, The first MPPT scan based on the initial MPPT voltage lower limit using the first power conversion device includes: performing a first control on the operating voltage of the first power conversion device, wherein the first control includes controlling the operating voltage of the first power conversion device to decrease from the open-circuit voltage of the first photovoltaic module to the initial MPPT voltage lower limit. The second MPPT scan based on the target MPPT voltage lower limit using the first power conversion device includes: performing a second control on the operating voltage of the first power conversion device, wherein the second control includes controlling the operating voltage of the first power conversion device to decrease from the initial MPPT voltage lower limit to the target MPPT voltage lower limit.

10. The method according to claim 8, characterized in that, The number of the second power conversion devices is multiple, and the method further includes: The first power conversion device is controlled to replace at least one of the plurality of second power conversion devices to jointly construct the grid voltage reference value with the remaining second power conversion devices, wherein the remaining second power conversion devices are the second power conversion devices other than the at least one second power conversion device among the plurality of second power conversion devices; Based on the MPPT scan of the at least one second power conversion device, a first photovoltaic characteristic curve and a second photovoltaic characteristic curve of the at least one second photovoltaic module are obtained, wherein the at least one second photovoltaic module is connected to the input terminal of the at least one second power conversion device.

11. The method according to claim 8, characterized in that, The power conversion system further includes a grid-connected transformer and a grid-connected switch. A first terminal of the grid-connected transformer is connected to the output terminals of both the first and second power conversion devices. A second terminal of the grid-connected transformer is used to connect to the public power grid via the grid-connected switch. The method further includes: The grid-connected switch is disconnected, and the second power conversion device is controlled to run a virtual synchronous generator control strategy or a voltage and frequency control strategy so that the grid voltage reference value exists when the power conversion system is in off-grid operation.

12. A controller, characterized in that, Used to perform the method as described in any one of claims 8-11.

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